<script>
    var count = 0;
    function fib(n){
        count ++;
        if(n <= 2){
            return 1;
        }
        return fib(n - 1) + fib(n - 2);
    }

    fib(5);
    console.log(count);//9
    count = 0;
    fib(6);
    console.log(count);//15
    count = 0;
    fib(20);
    console.log(count);//13529
    count = 0;
    fib(21);
    console.log(count);//21891
</script>